Semiconductor manufacturing method of die pick-up from wafer

ABSTRACT

A manufacturing method of a semiconductor device comprising the steps of: affixing a die attach film and a dicing film to a back surface of a semiconductor wafer: thereafter dicing the semiconductor wafer and the die attach film to divide the semiconductor wafer into a plurality of semiconductor chips: thereafter pulling the dicing film from the center toward the outer periphery of the dicing film with a first tensile force to cut the die attach film chip by chip; and thereafter picking up the semiconductor chips together with the die attach film while pulling the dicing film from the center toward the outer periphery of the dicing film with a second tensile force smaller than the first tensile force.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationNo. 2005-013690 filed on Jan. 21, 2005, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device manufacturingtechnique and more particularly to a technique applicable effectively tothe step of dividing a semiconductor wafer affixed to a dicing film intoplural semiconductor chips and thereafter separating the semiconductorchips from the dicing film and the step of mounting each of theseparated semiconductor chips onto a mounting base through a die bondingpaste.

As steps included in a process of mounting a semiconductor chip(hereinafter referred to simply as “chip”) onto a mounting base such asa wiring substrate or a lead frame to assemble a semiconductor packagethere are a pickup step of separating a chip from a semiconductor wafer(simply as “wafer” hereinafter) and picking it up and a pellet bondingstep of bonding the picked-up chip onto a mounting base. The pickup stepand the subsequent pellet bonding step are also called die bondingsteps.

In the pickup step, dicing is performed in a state in which a dicingtape is affixed to the back surface of a wafer with a desired integratedcircuit formed thereon, to divide the wafer into plural chips. Next, thechips are peeled off one by one while pushing needles or the likeagainst the back surface of the dicing tape and thereafter each chip isconveyed onto a mounting base while being held by a collet.

In the next pellet bonding step, paste (adhesive) for die bonding is fedonto the mounting base with use of a paste applicator called a dispenserand the chip is bonded onto the mounting base through the paste. Therealso is known a method wherein a filmy adhesive material called dieattach film is affixed beforehand to the back surface of a semiconductorchip and the chip is bonded onto a mounting base through the die attachfilm. In case of using the die attach film, dicing is performed whileaffixing both die attach film and dicing tape to the back surface of awafer in the pickup step. Then, the divided chips are separated from thedicing tape together with the die attach film and are each conveyed ontothe mounting base.

Japanese Unexamined Patent Publication No. 2004-146727 (PatentLiterature 1) discloses a technique wherein a wafer on a wafer sheet isdiced and thereafter the wafer sheet is expanded to divide the waferinto chips.

Japanese Unexamined Patent Publication No. Hei 11(1999)-214487 (PatentLiterature 2) discloses a technique of expanding a wafer sheet by meansof a locking ring.

Japanese Unexamined Patent Publication No. 2004-273895 (PatentLiterature 3) and No. 2004-193241 (Patent Literature 4) disclose atechnique of dividing a wafer affixed to a die attach film into chips.

Japanese Unexamined Patent Publication No. 2000-265943 (PatentLiterature 5) and No. 2002-346462 (Patent Literature 6) disclose atechnique wherein a scraping groove is formed in the outer periphery ofa sealing disc connected to a plunger pump of a paste applicator, andpaste leaking from a sealing surface is collected into the scrapinggroove, thereby improving the adhesion of the sealing surface.

Japanese Unexamined Patent Publication No. Hei 9(1997)-162205 (PatentLiterature 7) discloses a technique of detecting a state of contact ornon-contact between a dicing table and a needle with use of an electriccircuit using a magnet.

Japanese Unexamined Patent Publication No. Hei 11(1999)-344536 (PatentLiterature 8), No. Hei 11(1999)-183573 (Patent Literature 9), No.2000-147063 (Patent Literature 10) and No. 2003-121494 (PatentLiterature 11) disclose a technique of semi-fixing a probe testingneedle with a magnet.

SUMMARY OF THE INVENTION

The present inventors found out that the following problems wereinvolved in the die bonding step using a die attach film and the diebonding step using a paste applicator.

The dicing tape used in dicing a wafer is composed of a tape base and anultraviolet curing type pressure-sensitive adhesive applied to onesurface of the tape base. Therefore, when ultraviolet light is radiatedto the back surface of the dicing tape to cure the pressure-sensitiveadhesive after dicing the wafer affixed to the dicing tape, the adhesiveforce of the pressure-sensitive adhesive becomes weaker and chipsbecomes easier to peel off from the dicing tape.

However, in the case where a die attach film is interposed between thewafer and the dicing tape, the adjacent die attach film cannot beseparated completely even if full-cut dicing is performed and hence thechips are difficult to peel off from the dicing tape.

Moreover, with a die attach film interposed between the wafer and thedicing tape, the die attach film is subsequently cut by stretching thedicing tape. It is optional whether the die attach film is to beseparated or not when dicing the wafer. However, if a chip is picked upin a stretched state of the dicing tape, the chip may be cracked underthe imposition of a local stress thereon. Particularly, as a recenttendency, chips formed thin to a thickness of several ten μm or so areoften used to promote a high-density semiconductor packaging, thusgiving rise to the problem that a crack is apt to occur at the time ofpickup.

In the pickup step for chips, the chips are separated one by one whilepushing needles or the thin rod against the back surface of the dicingtape. However, it is necessary to increase the number of needles whenthe chip size is large or decrease the number of needles when the chipsize is small. That is, it is necessary to provide plural types ofpickup devices according to chip sizes, thus giving rise to the problemthat the equipment cost of the pickup step increases.

Next, a description will be given below about problems involved in thedie bonding step using a paste applicator. FIG. 34 is a sectional viewshowing a principal portion of a dispenser which the present inventorshave studied (not a prior art), FIG. 35 is a plan view of a sealing discas a mechanical part of the said dispenser, and FIG. 36 is a sectionalview taken along line A-A in FIG. 35.

The dispenser, indicated at 60, includes a main cylinder 61, a cap 62bolted to a lower end of the main cylinder 61, and a block 63 bolted toa lower end of the cap 62. A suction port 64 and a discharge port 65 areformed in the interior of the block 63. A nozzle 66 is connected to alower end portion of the discharge port 65 and a syringe 68 is connectedto one end portion of the suction port 64 through a liquid feed tube 67.The syringe 68 is a tank for the storage of die bonding paste and has astructure to transport the paste to the suction port 64 with airpressure for example.

A cylindrical sealing disc formed of a synthetic resin or a ceramicmaterial is fixed in the interior of the cap 62. The sealing disc 69 hasa suction hole 70 which is put in communication with an opposite endportion of the suction port 64 and a discharge hole 71 which is put incommunication with an upper end portion of the discharge port 65. Agenerally arcuate suction groove 72 connected to the suction hole 70 isformed in part of an upper surface of the sealing disc 69 and agenerally arcuate discharge groove 73 connected to the discharge hole 71is formed in another part of the upper surface of the sealing disc 69.

A cylindrical valve disc 74 formed of a ceramic material is disposed ontop of the sealing disc 69 so as to be rotatable while a lower surfacethereof slides relative to the upper surface of the sealing disc 69. Forexample three plunger inserting holes 75 are formed through both upperand lower surfaces of the valve disc 74.

For example three plungers 76 are disposed above the valve disc 74vertically movably, lower end portions of the plungers 76 being insertedrespectively into the plunger inserting holes 75.

When the valve disc 74 is rotated by a rotational drive means such as amotor (not shown), the rotation of the motor is converted to a verticalmotion by a cam mechanism (not shown) and the plungers 76 advance orretreat vertically within the plunger inserting holes 75 of the valvedisc 74.

When a plunger inserting hole 75 moves to a position overlapping thesuction groove 72 of the sealing disc 69 with rotation of the valve disc74, the associated plunger rises and a negative pressure is developedwithin the plunger inserting hole 75. Consequently, the paste storedwithin the syringe 68 passes through the liquid feed tube 67, suctionport 64, suction hole 70 and suction groove 72 and is sucked into theplunger inserting hole 75. Next, when the valve disc 74 further rotatesand the plunger inserting hole 75 moves to a position overlapping thedischarge groove 73, the plunger 76 moves down and causes the pastepresent within the plunger inserting hole 75 to be discharged from thetip of the nozzle 66 through the discharge groove 73, discharge hole 71and discharge port 65. In this way, the suction and discharge of pasteare repeated each time the valve disc 74 rotates once, whereby apredetermined amount of paste is applied to a predetermined position ofthe mounting base.

According to the structure of the dispenser 60 described above, thevalve disc 74 rotates while its lower surface slides relative to theupper surface of the sealing disc 69. Therefore, if a slight gap isformed between the sliding surfaces due to variations in flatness ofeach of lower surface of the valve disc 75 and the upper surface of thesealing disc 69, a portion of the paste present in the interior of theplunger inserting hole 75 and the interior of the discharge groove 73which have been pressurized by the downward movement of the plunger 76leaks out to the sliding surfaces.

However, since a hard filler such as silicon filler is contained in thedie bonding paste, if the valve disc 74 is continued to rotate in aleaking state of the paste to the sliding surfaces, the sliding surfacesbecome worn and the gap becomes wider, so that the paste leaks out tothe exterior of the sliding surfaces. As a result, there arises theproblem that the amount of the paste fed to the mounting base varies.Besides, if the paste leaking out to the exterior of the slidingsurfaces solidifies in the interior of the dispenser 60, a smoothoperation of mechanical parts is obstructed, thus giving rise to theproblem that the maintenance frequency for the dispenser 60 increases.

It is an object of the present invention to provide a die bondingtechnique whereby chips affixed to a dicing tape through a die attachfilm can be picked up rapidly without the occurrence of cracking orchipping of a semiconductor chip.

It is another object of the present invention to provide a techniquewhich permits the reduction of cost of a device for picking up chipsaffixed to a dicing tape.

It is a further object of the present invention to provide a die bondingtechnique which permits a die bonding paste to be fed in a stable amountonto a mounting base.

The above and other objects and novel features of the present inventionwill become apparent from the following description and the accompanyingdrawings.

The following is an outline of typical modes of the present invention asdisclosed herein.

-   (1) A manufacturing method of a semiconductor device comprises the    steps of:-   (a) affixing a die attach film and a dicing film to a back surface    of a semiconductor wafer;-   (b) after the step (a), dicing the semiconductor wafer and the die    attach film to divide the semiconductor wafer into a plurality of    semiconductor chips;-   (c) after the step (b), pulling the dicing film from the center    toward the outer periphery of the dicing film with a first tensile    force to cut the die attach film chip by chip; and-   (d) after the step (c), picking up the semiconductor chips together    with the die attach film while pulling the dicing film from the    center toward the outer periphery of the dicing film with a second    tensile force smaller than the first tensile force.-   (2) A manufacturing method of a semiconductor device comprises the    steps of:-   (a) affixing a dicing film to a back surface of a semiconductor    wafer;-   (b) after the step (a), dicing the semiconductor wafer to divide the    semiconductor wafer into a plurality of semiconductor chips; and-   (c) after the step (b), sticking up a back surface of the dicing    film by a stick-up means of a pickup means to suck the stuck-up    semiconductor chips to a collet, thereby separating the    semiconductor chips from the dicing film,

the stick-up means comprising a plurality of needles having tips forcontact with the back surface of the dicing film, a magnet for fixingrear ends of the plural needles, and a needle holder which includes aplurality of through holes each having an inside diameter slightlylarger than the diameter of each of the needles and controls theattitude of the needles inserted respectively into the plural throughholes.

-   (3) A manufacturing method of a semiconductor device comprises the    steps of:-   (a) affixing a dicing film to a back surface of a semiconductor    wafer;-   (b) after the step (a), dicing the semiconductor wafer to divide the    semiconductor wafer into a plurality of semiconductor chips;-   (c) after the step (b), sticking up a back surface of the dicing    film by a stick-up means of a pickup means to suck the stuck-up    semiconductor chips to a collet, thereby separating the    semiconductor chips from the dicing film;-   (d) feeding a die bonding paste discharged from a nozzle of a    dispenser onto a mounting base; and-   (e) mounting the semiconductor chips picked up in the step (c) onto    the mounting base through the paste,

the dispenser comprising a block having a suction port and a dischargeport, a sealing disc fixed onto the block and having a lower surface incontact with the block and an upper surface opposite to the lowersurface, a valve disc having a lower surface adapted to rotate insliding abutment against the upper surface of the sealing disc andfurther having a plunger inserting hole formed through both upper andlower surfaces thereof, and a plunger supported vertically movably andinserted partially into the plunger inserting hole of the valve disc,

the sealing disc having a suction hole which is put in communicationwith the suction port of the block and a discharge hole which is put incommunication with the discharge port of the block,

the upper surface of the sealing disc being formed with a suction grooveconnected to the suction hole and a discharge groove connected to thedischarge hole,

the suction groove being formed in the upper surface of the sealing discso as to surround the entire periphery of the discharge groove.

-   (4) A manufacturing method of a semiconductor device comprises the    steps of:-   (a) affixing a die attach film and a dicing film to a back surface    of a semiconductor wafer;-   (b) after the step (a), dicing the semiconductor wafer and the    dicing film to divide the semiconductor wafer into a plurality of    semiconductor chips;-   (c) after the step (b), pulling the dicing film from the center    toward the outer periphery of the dicing film with a first tensile    force to cut the die attach film chip by chip; and-   (d) after the step (c), while pulling the dicing film from the    center toward the outer periphery of the dicing film with a second    tensile force smaller than the first tensile force, sticking up a    back surface of the dicing film by a stick-up means of a pickup    means to suck the stuck-up semiconductor chips to a collet, thereby    separating the semiconductor chips together with the die attach film    from the dicing film,

the stick-up means comprising a plurality of needles having tips forcontact with the back surface of the dicing film, a magnet for fixingrear ends of the plural needles, and a needle holder which includes aplurality of through holes each having an inside diameter slightlylarger than the diameter of each of the needles and controls theattitude of the needles inserted respectively into the plural throughholes, the semiconductor chips being stuck up from the back surface ofthe dicing film using the stick-up means.

-   (5) A semiconductor manufacturing apparatus comprises a pickup means    for sticking up, using the stick-up means, a back surface of a    dicing film with a plurality of semiconductor chips bonded to a    surface thereof to suck the stuck-up semiconductor chips to a    collet, thereby separating the plural semiconductor chips from the    dicing film,

wherein the stick-up means comprises a plurality of needles having tipsfor contact with the back surface of the dicing film, a magnet forfixing rear ends of the plural needles, and a needle holder whichincludes a plurality of through holes each having an inside diameterslightly larger than the diameter of each of the needles and controlsthe attitude of the needles inserted respectively into the pluralthrough holes.

-   (6) In the above semiconductor manufacturing apparatus, the number    of the needles attached to the pickup means is increased or    decreased in accordance with the size of each of the semiconductor    chips.-   (7) In the above semiconductor manufacturing apparatus, a portion of    the plural needles and the remaining portion thereof are adapted to    vertically move independently of each other.-   (8) A manufacturing method of a semiconductor device comprises the    step of feeding a die bonding paste discharged from a nozzle of a    dispenser onto a mounted base and thereafter mounting semiconductor    chips onto the mounting base through the paste,

the dispenser comprising a block having a suction port and a dischargeport, a sealing disc fixed onto the block and having a lower surface incontact with the block and an upper surface opposite to the lowersurface, a valve disc having a lower surface adapted to rotate insliding abutment against the upper surface of the sealing disc andfurther having a plunger inserting hole formed through both upper andlower surfaces thereof, and a plunger supported vertically movably andinserted partially into the plunger inserting hole of the valve disc,

the sealing disc having a suction hole which is put in communicationwith the suction port of the block and a discharge hole which is put incommunication with the discharge port of the block,

the upper surface of the sealing disc being formed with a suction grooveconnected to the suction hole and a discharge groove connected to thedischarge hole,

the suction groove being formed in the upper surface of the sealing discso as to surround the entire periphery of the discharge groove.

-   (9) In the above manufacturing method of a semiconductor device, in    the upper surface of the sealing disc, the suction groove is the    widest near the suction hole and is narrow in a region spaced away    from the suction hole.-   (10) In the above manufacturing method of a semiconductor device,    the suction hole of the sealing disc is formed through the sealing    disc in a portion positioned on the center side with respect to the    discharge hole, while the discharge hole is formed through the    sealing disc in a direction perpendicular to both upper and lower    surfaces of the sealing disc in a portion positioned on an outer    side with respect to the suction hole.-   (11) In the above manufacturing method of a semiconductor device,    the sealing disc is formed of a ceramic material and the upper    surface thereof is formed to a flatness of not larger than 0.001 mm.-   (12) In the above manufacturing method of a semiconductor device, an    edge for recovery of the paste leaking out onto the upper surface of    the sealing disc is formed in a side wall surface of the sealing    disc.-   (13) A manufacturing method of a semiconductor device comprises the    steps of:-   (a) affixing a dicing film to a back surface of a semiconductor    wafer;-   (b) after the step (a), dicing the semiconductor wafer to divide the    semiconductor wafer into a plurality of semiconductor chips;-   (c) after the step (b), sticking up a back surface of the dicing    film by a stick-up means of a pickup means to suck the stuck-up    semiconductor chips to a collet, thereby separating the    semiconductor chips from the dicing film;-   (d) feeding a die bonding paste discharged from a nozzle of a    dispenser to a mounting base; and-   (e) mounting the semiconductor chips stuck up in the step (c) onto    the mounting base through the paste,

the dispenser comprising a block having a suction port and a dischargeport, a sealing disc fixed onto the block and having a lower surface incontact with the block and an upper surface opposite to the lowersurface, a valve disc having a lower surface adapted to rotate insliding abutment against the upper surface of the sealing disc andfurther having a plunger inserting hole formed through both upper andlower surfaces thereof, and a plunger supported vertically movably andinserted partially into the plunger inserting hole of the valve disc,

the sealing disc having a suction hole which is put in communicationwith the suction port of the block and a discharge hole which is put incommunication with the discharge port of the block,

the upper surface of the sealing disc being formed with a suction grooveconnected to the suction hole and a discharge groove connected to thedischarge hole,

the suction groove of the sealing disc being formed in the upper surfaceof the sealing disc so as to surround the entire periphery of thedischarge groove.

-   (14) A semiconductor manufacturing apparatus comprises a dispenser    for feeding a die bonding paste onto a mounting base,

wherein the dispenser comprises a block having a suction port and adischarge port, a sealing disc fixed onto the block and having a lowersurface in contact with the block and an upper surface opposite to thelower surface, a valve disc having a lower surface adapted to rotate insliding abutment against the upper surface of the sealing disc andfurther having a plunger inserting hole formed through both upper andlower surfaces thereof, and a plunger supported vertically movably andinserted partially into the plunger inserting hole of the valve disc,

the sealing disc having a suction hole which is put in communicationwith the suction port of the block and a discharge hole which is put incommunication with the discharge port of the block,

the upper surface of the sealing disc being formed with a suction grooveconnected to the suction hole and a discharge groove connected to thedischarge hole,

the suction groove being formed in the upper surface of the sealing discso as to surround the entire periphery of the discharge groove.

-   (15) In the above semiconductor manufacturing apparatus, in the    upper surface of the sealing disc, the suction groove is the widest    near the suction hole and is narrow in a region spaced away from the    suction hole.-   (16) In the above semiconductor manufacturing apparatus, the suction    hole of the sealing disc is formed through the sealing disc in a    portion positioned on the center side with respect to the discharge    hole, while the discharge hole is formed through the sealing disc in    a direction perpendicular to both upper and lower surfaces of the    sealing disc in a portion positioned on an outer side with respect    to the suction hole.-   (17) In the above semiconductor manufacturing apparatus, the sealing    disc is formed of a ceramic material and the upper surface thereof    is formed to a flatness of not larger than 0.001 mm.-   (18) In the above semiconductor manufacturing apparatus, an edge for    recovery of the paste leaking out onto the upper surface of the    sealing disc is formed in a side wall surface of the sealing disc.

The following is a brief description of effects obtained by the typicalmodes of the present invention as disclosed herein.

The chips affixed to the dicing tape through the die attach film can bepicked up rapidly without the occurrence of cracking or chipping.

It is possible to reduce the cost of the device for picking up the chipsaffixed to the dicing tape.

The die bonding paste can be fed in a stable amount onto the mountingbase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of semiconductor chips used inmanufacturing a semiconductor device according to an embodiment of thepresent invention;

FIG. 2 is a side view showing a grinding step for a semiconductor wafer;

FIG. 3 is a side view showing a step of affixing a die attach film and adicing tape to the semiconductor wafer;

FIG. 4 is a perspective view showing a dicing step for the semiconductorwafer;

FIG. 5 is a side view showing the dicing step for the semiconductorwafer;

FIGS. 6( a), 6(b) and 6(c) are schematic sectional views each showing astate of the die attach film in the dicing step for the semiconductorwafer;

FIG. 7 is a perspective view showing an appearance of a pickup device;

FIG. 8 is a schematic plan view showing a positional relation between awafer ring and an expand ring in the pickup device;

FIG. 9 is a schematic sectional view showing a positional relation amongthe wafer ring, a support ring and the expand ring in the pickup device;

FIG. 10 is a partially broken-away perspective view showing a principalportion of a chucking element;

FIG. 11 is an enlarged perspective view of an upper surface and thevicinity thereof of the chucking element;

FIG. 12 is a partially broken-away sectional view of a pickup unitaccommodated in the interior of the chucking element;

FIG. 13 is an exploded perspective view of the pickup unit;

FIGS. 14( a) and 14(b) are side views each showing the number of needlesinserted into through holes formed in a needle holder;

FIG. 15 is a side view showing a chip pickup step;

FIG. 16 is a sectional view showing the chip pickup step;

FIG. 17 is a perspective view showing an appearance of a collet driveunit with a chucking collet loaded thereto;

FIG. 18 is a sectional view showing the chip pickup step;

FIG. 19 is a sectional view showing the chip pickup step;

FIG. 20 is a perspective view showing a pellet bonding step for eachchip;

FIG. 21 is a perspective view showing the pellet bonding step for chips;

FIG. 22 is a partially broken-away sectional view of a pickup unit,showing another example of a needle fixing method;

FIG. 23 is a partially broken-away sectional view of the pickup unit,showing a further example of a needle fixing method;

FIG. 24 is a sectional view showing another example of a chip pickupmethod;

FIG. 25 is a sectional view showing the another example of the chippickup method;

FIG. 26 is a sectional view showing the another example of the chippickup method;

FIG. 27 is a sectional view showing a further example of a chip pickupmethod;

FIG. 28 is a sectional view showing a still further example of a chippickup method;

FIG. 29 is a perspective view showing an appearance of a dispenser;

FIG. 30 is a plan view of a sealing disc as a mechanical part of thedispenser shown in FIG. 29;

FIG. 31 is a sectional view taken along line A-A in FIG. 30;

FIG. 32 is a perspective view showing a pellet bonding step for chips;

FIG. 33 is a perspective view showing the pellet bonding step for chips;

FIG. 34 is a sectional view showing a principal portion of a dispenserwhich the present inventors have studied;

FIG. 35 is a plan view of a sealing disc as a mechanical part of thedispenser shown in FIG. 34; and

FIG. 36 is a sectional view taken along line A-A in FIG. 35.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detailhereinunder with reference to the accompanying drawings. In all of thedrawings for illustrating the embodiments, the same members areidentified by the same reference numerals in principle, and repeatedexplanations thereof will be omitted.

First Embodiment

A manufacturing method of a semiconductor device according to a firstembodiment of the present invention will be described below step by stepwith reference to FIGS. 1 to 21.

First, there is provided such a wafer 1A of a single crystal silicon asshown in FIG. 1. A main surface of the wafer 1A is partitioned intoplural chip regions 1A′ and an integrated circuit is formed on each chipregion 1A′ by a known semiconductor manufacturing process.

Next, as shown in FIG. 2, a back grinding tape 3 for protection of theintegrated circuit is affixed to the main surface (the lower surfaceside in the figure) of the wafer 1A. In this state, grinding isperformed for a back surface (the upper surface side in the figure) ofthe wafer 1A by means of a grinder and thereafter a damaged layerresulting from the grinding is removed by such a method as wet etching,dry polishing or plasma etching, thereby thinning the wafer 1A to athickness of not larger than 100 μm, e.g., 30 to 60 μm.

Next, the back grinding tape 3 is peeled off from the main surface ofthe wafer 1A and thereafter, as shown in FIG. 3, a die attach film 4 isaffixed to the back surface of the wafer 1A, further, a dicing tape 5 isaffixed to a back surface of the die attach film 4. The die attach film4 is a filmy adhesive material of about 20 to 100 μm in thickness whichconstitutes an adhesive layer at the time of mounting chips obtained bylater-described dicing of the wafer 1A onto a mounting base such asother chips or a wiring substrate. The dicing tape 5 is a tape of about90 to 120 μm in thickness comprising a tape base such as polyolefin (PO)or polyvinyl chloride (PVC) and an ultraviolet curing typepressure-sensitive adhesive or the like applied to one surface of thetape base to impart tackiness to the tape.

Next, as shown in FIG. 4, a wafer ring 6 is bonded to an outer peripheryportion of a surface of the dicing table 5, then, as shown in FIG. 5,the wafer 1A is diced using a dicing blade 7 to divide the plural chipregions 1A′ into chips 1. At this time, the dicing tape 7 is not cut. Asto the die attach film 4, it is desirable for the same film to be cutcompletely. However, the die attach film 4 may be cut while allowing apart thereof to remain (half-cutting) as shown in FIG. 6( a), or onlythe wafer 1A may be diced without cutting the die attach film 4 as shownin FIG. 6( b), or only the wafer 1A may be half-cut and separated usinga pickup device which will be described later, as shown in FIG. 6( c).

Next, as shown in FIGS. 7 to 9, the dicing tape 5 with plural chips 1bonded thereto is positioned horizontally onto a support ring 11 of apickup device 10 and the wafer ring 6 bonded to the outer peripheryportion of the dicing tape 5 is held by an expand ring 12. FIG. 7 is aperspective view showing an appearance of the pickup device 10, FIG. 8is a schematic plan view showing a positional relation between the waferring 6 and the expand ring 12, and FIG. 9 is a schematic sectional viewshowing a positional relation among the wafer ring 6, support ring 11and expand ring 12. As shown in FIG. 9, a chucking element 13 forpushing up each chip 1 is disposed inside the support ring 11. With adrive mechanism (not shown), the chucking element 13 is adapted to movehorizontally and vertically.

FIG. 10 is a partially broken-away perspective view showing a principalportion of the chucking element 13 and FIG. 11 is an enlargedperspective view of an upper surface and the vicinity thereof of thechucking element 13. A pickup unit 1 is accommodated in the interior ofthe chucking element 13 which has a columnar contour. Plural needles 16for pushing up a back surface of the dicing tape 5 are provided on anupper surface of the pickup unit 15. With a drive mechanism (not shown),the pickup unit 15 is adapted to move vertically in the interior of thechucking element 13.

Plural suction holes 30 and plural concentric slots 31 are formed in theperipheral portion of the upper surface of the chucking element 13. Thesuction holes 30 also serve as holes for allowing the tips of theneedles 16 formed on the pickup unit 15 to project from the uppersurface of the chucking element 13. The interior of the chucking element13 is pressure-reduced by an evacuating mechanism, and when the chuckingelement 13 is raised to bring its upper surface into contact with theback surface of the dicing tape 5, the dicing tape back surface comesinto close contact with the chucking element upper surface.

FIG. 12 is a partial broken-away sectional view of the pickup unit 15accommodated in the interior of the chucking element 13 and FIG. 13 isan exploded perspective view of the pickup unit 15. The pickup unit 15is made up of plural needles 16, a permanent magnet 17, a magnet holder18 for holding the permanent magnet 17, and a needle holder 20 in whichthere are formed plural through holes 19 in a closely spaced manner. Theplural needles 16 are made of a magnetic material such as metalcontaining iron as a main component and are formed so as to be mutuallyequal in dimensions (length and diameter). The permanent magnet 17 has acontour in the shape of a rectangular parallelepiped and an uppersurface thereof is formed flat so as to have neither concaves norconvexes. The inside diameter of each of the through holes 19 formed inthe needle holder 20 is slightly larger than the diameter of each needle16 and the length thereof is shorter than the length of each needle 16.

For assembling the pickup unit 15, the permanent magnet 17 is fitted ina groove 21 formed in an upper surface of the magnet holder 18 andthereafter pins 22 erected on the upper surface of the magnet holder 18are inserted into guide holes 23 formed in the needle holder 20. Next,using tweezers or the like, the needles 16 are inserted one by one intothe through holes 19 of the needle holder 20. By so doing, rear ends ofthe plural needles 16 are attracted by the permanent magnet 17 and comeinto close contact with the upper surface of the permanent magnet;besides, the needles 16 are controlled to a vertical attitude by theneedle holder 20, so that their tips projecting from an upper surface ofthe needle holder 20 become equal in height.

The number of the needles 16 to be inserted into the through holes 19 ofthe needle holder 20 is increased or decreased in accordance with thesize of each of the chips to be picked up. More specifically, in thecase where the size of each chip 1 is large (FIG. 14( a)), the number ofthe needles 16 is increased, while in the case where the chip size issmall (FIG. 14( b)), the number of the needles 16 is decreased. In bothcases, the number of the needles 16 is adjusted so that the position ofthe outermost periphery portion of the plural needles 16 nearly overlapsthe outermost periphery portion of the chips 1. It is preferable thatthe spacing between adjacent needles 16 be as narrow as possible. If thespacing between adjacent needles 16 is wide, the interface between thedicing tape 5 and the die attach film 4 does not peel off uniformlythroughout the whole surface of each chip 1 at the time of picking upthe chip. Particularly in the case of a thin chip 1, there is a fear ofoccurrence of cracking or chipping.

Since the pickup unit 15 adopts a method wherein the needles 16 made ofa magnetic material are inserted into the through holes 19 and are fixedby the permanent magnet, it is characteristic in that the spacingbetween adjacent needles 16 can be made very narrow. In contrasttherewith, in case of adopting a method wherein the rear ends of theneedles 16 are fixed by clamping with screws or the like, the spacingbetween adjacent needles 16 becomes wide because the screw diameter islarger than the diameter of each needle 16.

The pickup unit 15 is further characteristic in that plural types ofchips 1 different in size can be picked up by only changing the numberof the needles 16 to be inserted into the through holes 19 of the needleholder 20. Thus, it is not necessary to provide plural types of pickupunits according to chip sizes and therefore it is possible to reduce thecost of the pickup device. Further, since the number of the needles 16to be inserted into the through holes 19 can be changed in a short timeby merely pulling out and inserting the needles 16, the pickup operationcan be done quickly even in such a manufacturing line as includes pluraltypes of chips 1 different in size.

For picking up each chip 1 on the dicing tape 5 established its positionin the pickup device 10, first ultraviolet light is radiated to thedicing tape 5. As a result, the pressure-sensitive adhesive applied tothe dicing tape 5 cures and its tackiness is deteriorated. Consequently,the interface between the dicing tape 5 and the die attach film 4becomes easier to peel off.

Next, as shown in FIG. 15, the expand ring 12 of the pickup device 10 isbrought down to push down the wafer ring 6 bonded to the outer peripheryportion of the dicing tape 5. As a result, the dicing tape 5 undergoes astrong tensile force acting from the center toward the outer peripheryportion of the dicing tape and is stretched horizontally withoutlooseness. Consequently, the die attach film 4 is also stretched at thesame time. As noted earlier, since the chip-to-chip region of the dieattach film 4 is half-cut in the dicing step for the wafer 1A, thestretched die attach film 4 is cut in this region and is separated chipby chip.

For cutting and separating the die attach film 4 it is preferable thatthe expand ring 12 be brought down at high speed and in a considerableamount to stretch the dicing tape 5 rapidly. In this embodiment, theexpand ring 12 is brought down 10 mm or more at a speed of, say, 10mm/sec though depending on the type of the dicing tape 5 and the dieattach film 4.

Next, for diminishing the horizontal tensile force exerted on the dicingtape 5, the expand ring 12 is raised slightly at low speed. The risingspeed and amount are, for example, lower than 10 mm/sec and about 3 mmor less, respectively. Subsequently, where required, the expand ring 12may be brought down for example at a low speed of lower than 10 mm/secand in an amount of about 3 mm or less and fine adjustment may be madeuntil the magnitude of the horizontal tensile force exerted on thedicing tape 5 reaches a level best suited to the pickup of each chip 1.

Next, as shown in FIG. 16, the chucking element 13 is moved to justunder the chip 1 to be peeled off and a chucking collet 33 is moved toabove the chip 1. A chucking hole 34 capable of being pressure-reducedin the interior thereof is formed centrally of the chucking collet 33 sothat only one chip 1 to be peeled off can be chucked and heldselectively. As shown in FIG. 17, the chucking collet 33 is attached toa lower end of a collet drive unit 35. The collet drive unit 35 issupported movably in both horizontal and vertical directions by a movingmechanism (not shown).

Next, as shown in FIG. 18, the chucking element 13 is raised, causingits upper surface to come into contact with the back surface of thedicing tape 5, and at the same time the internal pressure of thechucking element 13 is reduced. As a result, the dicing tape 5positioned under the chip 1 to be peeled off comes into close contactwith the upper surface of the chucking element 13. At the same time, thechucking collet 33 is brought down, causing its bottom to come intocontact with an upper surface of the chip 1, the the internal pressureof the chucking port 34 is reduced. As a result, the chip 1 to be peeledoff comes into close contact with the bottom of the chucking collet 33.

Next, as shown in FIG. 19, the pickup unit 15 installed within thechucking element 13 is raised to stick up the tips of the needles 16from the upper surface of the chucking element 13, thereby pushing upthe dicing tape 5. At the same time, the chucking collet 33 is pulledup, whereby the chip 1 is separated from the dicing tape 5 and is pulledup by the chucking collet 33.

If a strong horizontal tension is exerted on the dicing tape 5 whenpushing up the dicing tape by the tips of the needles 16 of the pickupunit 15, the dicing tape which underlies the chip adjacent to the chip 1to be peeled off is also pushed up simultaneously and thus there is afear that the chip 1 other than the chip to be peeled off may also beseparated from the dicing tape 5. In this embodiment, however, as notedpreviously, the horizontal tensile force exerted on the dicing tape 5 isset beforehand to a magnitude best suited to the pickup operation andthus there is no fear of separation of any other chip 1 than the chip 1to be peeled off from the dicing tape 5.

When pushing up the dicing tape 5 by the tips of the needles 16, if thespacing between adjacent needles 16 is wide, the interface between thedicing tape 5 and the die attach film 4 no longer peels off uniformlythroughout the whole surface of the chip 1 to be peeled off. Therefore,particularly in the case of a thin chip 1, there is a fear of occurrenceof cracking or chipping. In this embodiment, however, as noted earlier,the dicing tape 5 and the die attach film 4 can be separated from eachother uniformly because the spacing between adjacent needles 16 in thepickup unit 15 used in this embodiment is extremely narrow.

Then, with the collet drive unit 35, the chip 1 thus separated from thedicing tape 5 is conveyed to the next step (pellet bonding step) whilebeing chucked by the chucking collet 33. As shown in FIG. 20, the chip 1thus conveyed to the pellet bonding step is mounted onto a mounting basesuch as wiring substrate 40. When the collet drive unit 35 returns againto the pickup device 10, the next chip 1 is separated from the dicingtape 5 and is mounted onto the wiring substrate 40 in accordance withthe foregoing procedure. Subsequently, in accordance with the sameprocedure, a predetermined number of chips 1 are separated one by onefrom the dicing tape 5 and are mounted onto the wiring substrate 40(FIG. 21). Thereafter, the wiring substrate 40 is heated, allowing thedie attach film 4 bonded to the back surfaces of the chips 1 to curethermally. The pellet bonding step is now over.

In the pickup unit 15 shown in FIGS. 12 and 13 the rear ends of theneedles 16 made of a magnetic material and inserted into the throughholes 19 of the needle holder 20 are fixed by the permanent magnet 17.In this case, if magnetism is imparted also to the needle holder 20, itis possible to hold the needles 16 in a more positive manner. Further,for example as shown in FIG. 22, a wide flange 29 may be formed in anintermediate portion of each needle 16 to restrict the height of theneedle and a presser plate 24 may be fixed to the upper surface of theneedle holder 20, allowing each needle 16 to be held by both presserplate 24 and needle holder 20. The presser plate 24 is fixed to theneedle holder 20 with screws or the like, but may be fixed to the needleholder 20 by being magnetized. In this case, if magnetism is impartedalso to a holder 25 which supports the rear ends of the needles 16, theneedles 16 can be held in a more positive manner. In the case wheremagnetism is imparted to both holder 25 and presser plate 24, it ispossible to suppress the deterioration of coercive forces of the two.For example, as shown in FIG. 23, the rear ends of the needles 16 may besupported by thin rubber tubes 26 erected on an upper surface of theholder 25. Also in this case, if the presser plate 24 is fixed to theupper surface of the needle holder 20 or if magnetism is imparted to thepresser plate 24, it is possible to hold the needles 16 in a morepositive manner.

Although in this embodiment each chip 1 is stuck up using plural needles16 loaded to the pickup unit 15, there may be adopted, for example, sucha method as shown in FIG. 24 wherein each chip 1 is stuck up in twosteps using plural needles 16 and a stick-up block 27 disposed insidethe needles. This method is effective particularly in the case where theadhesion between the dicing tape 4 and the die attach film 4 is strongor in the case where the size of each chip 1 is large.

The block 27 is constituted by a columnar member larger in diameter thaneach needle 16 and a flat upper surface thereof comes into surfacecontact with the back surface of the dicing tape 5. The needles 16 andthe block 27 are vertically movable independently of each other, and ina stand-by condition the tips of the needles 16 and the upper surface ofthe block 27 are flush with each other. Also in this case, the number ofneedles 16 is increased when the size of the chip 1 to be peeled off islarge and it is decreased when the chip size is small.

For sticking up each chip 1 with use of the needles 16 and the block 27,the chucking element 13 is raised to bring its upper surface intocontact with the back surface of the dicing tape 5, as shown in FIG. 25,and the internal pressure of the chucking element 13 is reduced. As aresult, the dicing tape 5 which underlies the chip 1 to be peeled offcomes into close contact with the upper surface of the chucking element13. At the same time, the chucking collet 33 is brought down, causingits bottom to come into contact with the upper surface of the chip 1,and the internal pressure of the chucking port 34 is reduced, with theresult that the chip 1 to be peeled off comes into close contact withthe bottom of the chucking collet 33.

Next, as shown in FIG. 26, the pickup unit 15 installed within thechucking element 13 is raised and the tips of the needles 16 and theupper surface of the block 27 are stuck up simultaneously from the uppersurface of the chucking element 13, thereby pushing up the dicing tape5. The chip 1 peels off from the dicing tape 5 if the adhesion betweenthe dicing tape 5 and the die attach film 4 is weak or if the size ofthe chip 1 is small, so all that is required is pulling up the chuckingcollet 13 simultaneously with the stick-up motion.

On the other hand, if the adhesion between the dicing tape 5 and the dieattach film 4 is strong or if the size of the chip 1 is large, only theperipheral portion of the chip 1 separates from the dicing tape 5 andthe central portion thereof does not peel off. In this case, only theupper surface of the block 27 is further stuck up as shown in FIG. 27.By so doing, the peeling of the dicing tape 5 proceeds from theperipheral portion toward the central portion of the chip 1, so that bypulling up the chucking collet 33 the chip 1 peels off completely fromthe dicing tape 5 and is pulled up.

The number of the block 27 is not limited to one, but plural blocks of asmall diameter may be combined. Or, there may be adopted such aconfiguration as shown in FIG. 28 in which the central portion of asmall-diameter block 27 is formed hollowly and a block 28 of a smallerdiameter is disposed within the hollow portion in such a manner that thetwo blocks 27 and 28 can vertically move independently of each other. Inthis case there may be adopted a three-step stick-up method whereinfirst plural needles 16 and the two blocks 27, 28 are stuck upsimultaneously, then only the two blocks 27 and 28 are further stuck up,and thereafter only the central block 28 is further stuck up.

For example, of the plural needles 16 attached to the pickup unit 15,plural needles 16 which are in contact with the peripheral portion ofthe chip 1 and plural needles 16 which are in contact with the centralportion of the chip 1 may moved vertically and independently of eachother, whereby the foregoing two- or three-step pickup operation can beeffected with only plural needles 16.

The above two- or three-step pickup method is also applicable to apellet bonding method not using the die attach film 4, i.e., to the casewhere each chip 1 bonded directly to the surface of the dicing tape 5 ispicked up and conveyed to the pellet bonding step and is thereafterbonded onto the mounting base through an adhesive (paste) which has beenapplied beforehand onto the mounting base with use of a pasteapplicator. In this case, the chip 1 can be separated quickly from thedicing tape 5 also by adopting a method wherein an ultrasonic waveoscillator is attached to the pickup unit 15 and oscillating needles 16and blocks 27, 28 are stuck up under ultrasonic oscillation.

Second Embodiment

In the previous first embodiment a description was directed mainly tothe die bonding method for chips 1 with use of the die attach film 4,while in this second embodiment a description will be given about a diebonding method for chips 1 with use of paste.

FIG. 29 is a perspective view showing an appearance of a dispenser usedin this second embodiment, FIG. 30 is a plan view of a sealing disc as amechanical part of the dispenser, and FIG. 31 is a sectional view takenalong line A-A in FIG. 30.

In a dispenser 50 used in this second embodiment, only the structure ofa sealing disc 51 is different from the dispenser 60 shown in theforegoing FIGS. 34 to 36. Therefore, other mechanical parts will bereferred to using the same reference numerals as those of the mechanicalparts of the dispenser 60.

As shown in FIGS. 30 and 31, a suction hole 52 which is put incommunication with a suction port 64 and a discharge hole 53 which isput in communication with a discharge port 65 are formed in the interiorof a cylindrical sealing disc 51. Further, a suction groove 54 connectedto the suction hole 52 and a discharge groove 55 connected to thedischarge hole 53 are formed in an upper surface of the sealing disc 51.

The position and shape of the discharge groove 55 are the same as thoseof the discharge groove 73 formed in the sealing disc 69 of thedispenser 60. On the other hand, the suction groove 54 is characterizedby being formed in the upper surface of the sealing disc 51 so as tosurround the entire periphery of the discharge groove 55.

As a result of the change in position and shape of the suction groove54, the position of the suction hole 52 connected to the suction groove54 is also changed in the upper surface of the sealing disc 51. Thesuction groove 54 is the widest near the suction hole 52 and is narrowin the region spaced away from the suction hole 52. The suction hole 52is put in communication with the suction port 64 in a lower surface ofthe sealing disc 51 and therefore the position of the suction hole 52 inthe lower surface of the sealing disc 51 is the same as the position ofthe discharge groove 73 formed in the sealing disc 69. Thus, as shown inFIG. 31, the suction hole 52 extends obliquely through the upper andlower surfaces of the sealing disc 51. On the other hand, the dischargehole 53 extends through the sealing disc 51 perpendicularly to bothupper and lower surfaces of the sealing disc. It is not always necessaryfor the suction hole 52 to extend obliquely through the sealing disc 51.The suction hole 52 may be formed through and perpendicularly to bothupper and lower surfaces of the sealing disc 51 in the portionpositioned on the center side with respect to the discharge hole 53.

The sealing disc 51 is formed of a ceramic material to improve theflatness of its upper and lower surfaces. Particularly, the uppersurface of the sealing disc 51 on which the lower surface of the valvedisc 74 slides is formed to a flatness (difference in height betweenconvex and concave portions) of not larger than 0.001 mm. Consequently,a portion of the paste present within the plunger inserting holes 75 andthe discharge groove 55 is difficult to leak out onto the slidingsurfaces, so that it is possible suppress wear of the sliding surfacescaused by a filler or the like contained in the paste.

Moreover, since the entire periphery of the discharge groove 55 issurrounded by the suction groove 54 in the upper surface of the sealingdisc 51, even if a portion of the paste present within the plungerinserting holes 75 and the discharge groove 55 should leak out to thesliding surfaces, the leaking paste can be recovered into the suctiongroove 54 and therefore it is possible to prevent the occurrence of suchan inconvenience as leaking-out of the paste to the exterior of thesliding surfaces.

Thus, since the leakage of the paste to the sliding surfaces of thevalve disc 74 and the sealing disc 51 can be prevented by attaching thesealing disc 51 according to this embodiment to the dispenser 50, it ispossible to prevent a change in the amount of the paste fed onto themounting base. Besides, since the paste leaking out to the exterior ofthe sliding surfaces can be prevented from solidifying in the interiorof the dispenser 50, it is possible to decrease the maintenancefrequency of the dispenser 50. To further ensure the prevention ofsolidifying of the paste leaking out to the exterior of the slidingsurfaces, an edge 56 may be formed in a side wall surfaces of thesealing disc 51 as shown in FIGS. 30 and 31 so that the paste leakingout to the sliding surfaces outside the suction groove 54 is recoveredinto the edge 56.

For pellet bonding of chips 1 onto a wiring substrate with use of a diebonding paste, first, as shown in FIG. 32, the paste, indicated at 57,is fed to a chip mounting region on the wiring substrate indicated at 41by means of the dispenser 50. Next, as shown in FIG. 33, each chip 1separated from the dicing tape is chucked and held by the chuckingcollet 33 in the collet drive unit 35 and is conveyed onto the wiringsubstrate 41, then is put on the paste 57. For separating the chip 1from the dicing tape there is used, for example, the pickup device ofthe multi-step stick-up type which has been shown in FIGS. 24 to 28 inconnection with the previous first embodiment.

Thereafter, the wiring substrate 41 is heated to cure the paste 57thermally, whereby the pellet bonding step is completed.

Although the present invention has been described above by way ofembodiments thereof, it goes without saying that the present inventionis not limited to the above embodiments, but that various changes may bemade within the scope not departing from the gist of the invention.

The pickup method described in the above embodiments, which involvesstretching the dicing tape with a strong tensile force to cut andseparate the die attach film, subsequently diminishing the tensionimposed on the dicing tape and then picking up each chip, is applicablenot only to the case where each chip is picked by sticking up the backsurface of the dicing tape with needles or a block but also, forexample, to the case where the chucking element is brought into contactwith the back surface of the dicing tape to suck the dicing tapedownward and each chip is picked up by the chucking collet.

Although in the above embodiments a description has been directed to thecase where the present invention is applied to the die bonding of chipseach formed thin to a thickness of about 30 to 60 μm, the presentinvention may be applied to the die bonding of thick chips each 60 μm ormore in thickness.

The present invention is a technique applicable effectively to a diebonding step which is one step included in a semiconductor manufacturingprocess.

1. A manufacturing method of a semiconductor device comprising the stepsof: (a) affixing a die attach film and a dicing film to a back surfaceof a semiconductor wafer, the dicing film having a center and an outerperiphery; (b) after the step (a), dicing the semiconductor wafer andthe die attach film to divide the semiconductor wafer into a pluralityof semiconductor chips; (c) after the step (b), pulling the dicing filmfrom the center toward the outer periphery of the dicing film with afirst tensile force to cut the die attach film chip by chip; and (d)after the step (c), picking up the semiconductor chips together with thedie attach film while pulling the dicing film from the center toward theouter periphery of the dicing film with a second tensile force smallerthan the first tensile force.
 2. A manufacturing method of asemiconductor device according to claim 1, wherein the dicing film ispulled in step (c) so as to increase tensile force on the dicing film tothe first tensile force, and wherein a speed of pulling the dicing filmso as to increase tensile force on the dicing film to the first tensileforce is higher than a speed of relaxing the pull on the dicing film soas to diminish the tensile force from the first to the second tensileforce.
 3. A manufacturing method of a semiconductor device according toclaim 1, wherein the thickness of each of the semiconductor chips is 30to 60 μm.
 4. A manufacturing method of a semiconductor device accordingto claim 1, further comprising the step of bonding the picked-upsemiconductor chips over a mounting base through the die attach film. 5.A manufacturing method of a semiconductor device according to claim 1,wherein at least a part of the die attach film remains uncut at the timeof dicing the semiconductor wafer and the die attach film in the step(b),and the die attach film is cut completely at the time of pulling thedicing film with the first tensile force in the step (c).
 6. Amanufacturing method of a semiconductor device according to claim 1,wherein, at the time of picking up the semiconductor chips in the step(d), a back surface of the dicing film is stuck up by a stick-up meansof a pickup means, the stick-up means sticking up the semiconductorchips to provide stuck-up semiconductor chips, the pickup means suckingthe stuck-up semiconductor chips, thereby separating the die attach filmpositioned on a back surface side of the semiconductor chips from thedicing film.
 7. A manufacturing method of a semiconductor deviceaccording to claim 1, including the further step, between step (c) andstep (d), of diminishing the tensile force from said first tensile forceto said second tensile force.
 8. A manufacturing method of asemiconductor device according to claim 7, wherein the dicing film ispulled in step (c) so as to increase tensile force on the dicing film tothe first tensile force, and wherein a speed of increasing tensile forceon the dicing film to said first tensile force is greater than a speedof diminishing tensile force from the first tensile force to the secondtensile force.